Scatterometer and Method of Scatterometry Using Acoustic Radiation

ABSTRACT

An acoustic scatterometer has an acoustic source operable to project acoustic radiation onto a periodic structure and formed on a substrate. An acoustic detector is operable to detect the −1st acoustic diffraction order diffracted by the periodic structure and while discriminating from specular reflection (0th order). Another acoustic detector is operable to detect the +1st acoustic diffraction order diffracted by the periodic structure, again while discriminating from the specular reflection (0th order). The acoustic source and acoustic detector may be piezo transducers. The angle of incidence of the projected acoustic radiation and location of the detectors and are arranged with respect to the periodic structure and such that the detection of the −1st and +1st acoustic diffraction orders and discriminates from the 0th order specular reflection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/162,009, filed Oct. 16, 2018, which claims priority of EuropeanApplication No. 17196893, filed on Oct. 17, 2017, and are incorporatedherein in their entirety by reference.

FIELD

The present disclosure relates to a scatterometer and method ofscatterometry usable, for example, in the manufacture of devices bylithographic techniques.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k1 lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k1×λ/NA,where λ is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k1 is an empirical resolution factor.In general, the smaller k1 the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof optical scatterometers have been developed for use in thelithographic field. These devices direct a beam of electromagneticradiation onto a target and measure one or more properties of thescattered electromagnetic radiation—e.g., intensity at a single angle ofreflection as a function of wavelength; intensity at one or morewavelengths as a function of reflected angle; or polarization as afunction of reflected angle—to obtain a diffraction “spectrum” fromwhich a property of interest of the target can be determined.

There are limitations in the performance of optical scatterometers. Forexample, for controlling the manufacture of semiconductor devices suchas 3D XPoint non-volatile memory and 3D NAND, it is difficult orimpossible to measure overlay through opaque mask layers that separatethe overlaid upper pattern from the lower pattern. The opaque layers maybe metal layers of several 10s of nm in thickness and carbon hardmasksof several μm in thickness. Metrology using optical scatterometers ischallenging as the masks employed are barely transmissive forelectromagnetic radiation, with the extreme case being metal masks,where electromagnetic radiation is absorbed and does not go through themetal mask at all.

SUMMARY

It is desirable to have an alternative to optical scatterometry, forexample to determine substrate properties such as overlay errors whenoptical scatterometry is not practical because of optically opaque orattenuating materials being present on the substrate.

According to a first aspect of the present invention, there is provideda scatterometer comprising:

an acoustic source operable to project acoustic radiation onto aperiodic structure formed on a substrate; and

an acoustic detector operable to detect an acoustic diffraction orderdiffracted by the periodic structure while discriminating from specularreflection,

wherein the scatterometer is operable to determine a property of thesubstrate based on the detected acoustic diffraction order.

According to a second aspect of the present invention, there is providedmethod of scatterometry comprising:

projecting acoustic radiation onto a periodic structure formed on asubstrate;

detecting an acoustic diffraction order diffracted by the periodicstructure while discriminating from specular reflection; and

determining a property of the substrate based on the detected acousticdiffraction order.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts, in schematic form, different configurations of acoustictransducers that may be used in embodiments of the present invention;

FIG. 5 depicts, in schematic form, a scatterometer in accordance with anembodiment of the present invention;

FIGS. 6 and 7 depict a simulated pressure distribution for a symmetricalstructure and an asymmetrical structure respectively;

FIG. 8 depicts, in schematic form, a simplified model to explain signalformation for conventional optical diffraction-based overlay;

FIG. 9 is a graph of a conventional optical swing curve;

FIG. 10 depicts, in schematic form a simplified model to explain signalformation for acoustic diffraction-based overlay;

FIG. 11 is a graph of an acoustic swing curve;

FIG. 12 depicts, in schematic form, an acoustic scatterometer with threeacoustic transducers including a central acoustic source, in accordancewith an embodiment of the present invention;

FIG. 13 depicts, in schematic form, an acoustic scatterometerimplemented with a phased array of transducers, in accordance with anembodiment of the present invention;

FIGS. 14a and 14b depict, in schematic form, an acoustic scatterometerwith three acoustic transducers including a central acoustic detector,in accordance with an embodiment of the present invention;

FIGS. 15a and 15b depict, in schematic form, an acoustic scatterometerwith two transducers operating as transceivers, in accordance with anembodiment of the present invention;

FIGS. 16a and 16b depict, in schematic form, an acoustic scatterometerwith one transducer operating as a transceiver, in accordance with anembodiment of the present invention;

FIG. 17 depicts a method in accordance with an embodiment of the presentinvention; and

FIG. 18 depicts a method including projecting acoustic radiation atdifferent times and with first and second angles of incidence, inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition an electromagneticradiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), asupport structure (e.g., a mask table) T constructed to support apatterning device (e.g., a mask) MA and connected to a first positionerPM configured to accurately position the patterning device MA inaccordance with certain parameters, a substrate table (e.g., a wafertable) WT constructed to hold a substrate (e.g., a resist coated wafer)W and connected to a second positioner PW configured to accuratelyposition the substrate in accordance with certain parameters, and aprojection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., comprising one ormore dies) of the substrate W.

In operation, the illuminator IL receives a radiation beam from aradiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation. Theilluminator IL may be used to condition the radiation beam B to have adesired spatial and angular intensity distribution in its cross sectionat a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, or forother factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus may be of a type wherein at least a portionof the substrate may be covered by a liquid having a relatively highrefractive index, e.g., water, so as to fill a space between theprojection system and the substrate—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253 and in PCT publication No. WO99-49504, whichare incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two (dualstage) or more substrate tables WT and, for example, two or more supportstructure T (not shown). In such “multiple stage” machines theadditional tables/structures may be used in parallel, or preparatorysteps may be carried out on one or more tables while one or more othertables are being used for exposing the design layout of the patterningdevice MA onto the substrate W.

In operation, the radiation beam B is incident on the patterning device(e.g., mask MA), which is held on the support structure (e.g., masktable T), and is patterned by the patterning device MA. Having traversedthe mask MA, the radiation beam B passes through the projection systemPS, which focuses the beam onto a target portion C of the substrate W.With the aid of the second positioner PW and position sensor IF (e.g.,an interferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g., so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and possibly another positionsensor (which is not explicitly depicted in FIG. 1) may be used toaccurately position the mask MA with respect to the path of theradiation beam B. Mask MA and substrate W may be aligned using maskalignment marks M1, M2 and substrate alignment marks P1, P2. Althoughthe substrate alignment marks as illustrated occupy dedicated targetportions, they may be located in spaces between target portions (theseare known as scribe-lane alignment marks).

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, one or more inspection tool MTmay be included in the lithocell LC. If errors are detected,adjustments, for example, may be made to exposures of subsequentsubstrates or to other processing steps that are to be performed on thesubstrates W, especially if the inspection is done before othersubstrates W of the same batch or lot are still to be exposed orprocessed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3. One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3in the third scale SC3).

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Tools to make such measurement are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of optical scatterometer metrology tools MT. Opticalscatterometers are versatile instruments which allow measurements of theparameters of a lithographic process by having a sensor in the pupil ora conjugate plane with the pupil of the objective of the scatterometer,measurements usually referred as pupil based measurements, or by havingthe sensor in the image plane or a plane conjugate with the image plane,in which case the measurements are usually referred as image or fieldbased measurements. Such scatterometers and the associated measurementtechniques are further described in patent applications US20100328655,US2011102753A1, US20120044470A, US20110249244, US20110026032 orEP1,628,164A, incorporated herein by reference in their entirety.Aforementioned scatterometers may measure gratings using light from softx-ray, visible light to near-IR wavelength range.

The scatterometer MT may be an angular resolved scatterometer. In such ascatterometer reconstruction methods may be applied to the measuredsignal to reconstruct or calculate properties of the grating. Suchreconstruction may, for example, result from simulating interaction ofscattered radiation with a mathematical model of the target structureand comparing the simulation results with those of a measurement.Parameters of the mathematical model are adjusted until the simulatedinteraction produces a diffraction pattern similar to that observed fromthe real target.

Alternatively, the scatterometer MT may be a spectroscopic scatterometerMT. In such spectroscopic scatterometer MT, the radiation emitted by aradiation source is directed onto the target and the reflected orscattered radiation from the target is directed to a spectrometerdetector, which measures a spectrum (i.e. a measurement of intensity asa function of wavelength) of the specular reflected radiation. From thisdata, the structure or profile of the target giving rise to the detectedspectrum may be reconstructed, e.g. by Rigorous Coupled Wave Analysisand non-linear regression or by comparison with a library of simulatedspectra.

Alternatively, the scatterometer MT may be an ellipsometricscatterometer. The ellipsometric scatterometer allows for determiningparameters of a lithographic process by measuring scattered radiationfor each polarization states. Such metrology apparatus emits polarizedlight (such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationSer. Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968,12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410incorporated herein by reference in their entirety.

A metrology target may be an ensemble of composite gratings, formed by alithographic process, mostly in resist, but also after etch process forexample. Typically the pitch and line-width of the structures in thegratings strongly depend on the measurement optics (in particular the NAof the optics) to be able to capture diffraction orders coming from themetrology targets. The targets may be measured in an underfilled mode orin an overfilled mode. In the underfilled mode, the measurement beamgenerates a spot that is smaller than the overall target. In theoverfilled mode, the measurement beam generates a spot that is largerthan the overall target. In such overfilled mode, it may also bepossible to measure different targets simultaneously, thus determiningdifferent processing parameters at the same time.

Overall measurement quality of a lithographic parameter using a specifictarget is at least partially determined by the measurement recipe usedto measure this lithographic parameter. The term “substrate measurementrecipe” may include one or more parameters of the measurement itself,one or more parameters of the one or more patterns measured, or both.For example, if the measurement used in a substrate measurement recipeis a diffraction-based optical measurement, one or more of theparameters of the measurement may include the wavelength of theradiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US2016-0161863 and US patentapplication US 2016-0370717, incorporated herein by reference in theirentirety.

In embodiments of the present invention, an acoustic scatterometer maybe used to replace the optical scatterometer and its function in thelithographic cell of FIG. 2 or control environment of FIG. 3.

In the following disclosure, the term “radiation” is used to encompassall types of acoustic radiation, including ultrasonic radiation.

In embodiments of the present invention, overlay between overlappinggratings is measured as an intensity asymmetry between diffractedacoustic orders. Embodiments of the present invention may be used toimplement diffraction-based acoustic metrology, wherein periodicstructure asymmetry or overlay is determined from the asymmetry betweenacoustic diffracted orders.

The all-acoustic overlay measurement may be performed by detecting thescattered acoustic pressure distribution when two gratings or structuresplaced on top of each other are excited by an acoustic wave. Sinceacoustic waves also obey the wave equation (Section 4, below), similardiffraction phenomena can be observed. The diffracted acoustic wave willalso have an asymmetric distribution in the higher orders, should anoverlay between gratings be present. This can be detected by singletransducers or by an array of 2D (two-dimensional) transducers. Today'stechnology allows fabrication of 2D acoustic detectors in the tens ofmicron range.

Besides intensity data, phase data can be included in the measurements.In acoustics one can detect the actual phase of the sound waves if thefrequencies are in the MHz-GHz range. For frequencies above GHz wheredigital acquisition is no longer possible, phase retrieval is required.

FIG. 4 depicts, in schematic form, different configurations of acoustictransducers that may be used in embodiments of the present invention. Acurved piezo element 402 is shaped to produce a focus of acousticradiation 404. A phased-array piezo element 406 uses electronic delaysbetween activation of discrete sub-elements to achieve electronicfocusing at and detection from a focal point 408. An acoustic lens 410has a piezo element 412 and a shaped component 414 to achieve a focus416.

Acoustic microscopy uses very high or ultra-high-frequency ultrasound toimage structures. Ultrasound is defined as a sound having a frequencyabove 20 kHz. Acoustic microscopes may operate with ultrasound rangingfrom ranging from 5 MHz to beyond 400 MHz, and as high as 2 to 5 GHz.Acoustic microscopes are non-destructive. The acoustic radiationpenetrates solid materials and the reflection and transmissionproperties of the ultrasonic radiation are used to detect cracks, voids,delamination and other defects within solid samples.

Ultrasound at low frequencies penetrates deeper into materials comparedto ultrasound at high frequencies. Semiconductor devices are typicallyfabricated in layers on a substrate, therefore high-frequencyultrasound, which does not penetrate deeply, is suitable for imagingdefects in semiconductor devices. For measurement with embodiments ofthe present invention, periodic structures such as overlaid gratings maybe very close to and at the surface of the substrate. Therefore theacoustic radiation used in accordance with such embodiments of thepresent invention may use ultra-high frequency ultrasound that does notpenetrate very deeply.

FIG. 5 depicts an acoustic scatterometer 502. The scatterometer has anacoustic source 520 operable to project acoustic radiation 526 onto aperiodic structure 538 and 540 formed on a substrate 536. The acousticsource is controlled by a processing unit 506, which outputs a controlsignal 512 to control acoustic projection by the acoustic source 520. Anacoustic detector 518 is operable to detect the −1st acousticdiffraction order 528 diffracted by the periodic structure 538 and 540while discriminating from specular reflection (0th order 532). Anotheracoustic detector 522 is operable to detect the +1st acousticdiffraction order 530 diffracted by the periodic structure, again whilediscriminating from the specular reflection (0th order 532). Theacoustic source and acoustic detector may be piezo transducers asdescribed with reference to FIG. 4. In the Figures, acoustic sources arelabelled “Tx” as in “transmit” and detectors are labelled “Rx” as in“receive”.

The angle of incidence of the projected acoustic radiation 526 andlocation of the detectors 518 and 522 are arranged with respect to theperiodic structure 538 and 540 such that the detection of the −1st and+1st acoustic diffraction orders 528 and 530 discriminates from the 0thorder specular reflection 532. In the examples described herein, theperiodic structure has a direction of periodicity perpendicular to thelines of the grating, i.e. from left to right in the cross-sectiondepicted in FIG. 5. The locations of the detectors are distributed alongthat direction, i.e. from left to right.

A couplant system provides a couplant 524 to facilitate transmission ofthe acoustic radiation between the acoustic source 520 and the acousticdetectors 518, 522 via the periodic structure 528 and 530. A couplantsystem similar to one used in known immersion techniques, as mentionedabove, may be used. A suitable couplant is distilled water.

The scatterometer 502 is operable to determine a property (in thisexample overlay error, OV) 504 of the substrate based on the detectedacoustic diffraction orders 528, 530. The scatterometer 502 is operableto determine the property 504 of the substrate based on at least one ofamplitude, phase and direction of one or more of the detected acousticdiffraction orders 528, 530. The property of the substrate may comprisesa physical property of the periodic structure, such as CD or overlay.

In this example, the scatterometer 502 is operable to determine astructural asymmetry 504 of the periodic structure 538 and 540 based onasymmetry of corresponding opposite detected (−1st 528 and +1st 530)acoustic diffraction orders diffracted by the periodic structure 538 and540. The acoustic detector 518 outputs an intensity signal I⁻¹ 514 whichis a measure of the amplitude of the −1st acoustic diffraction order514. The acoustic detector 522 outputs an intensity signal I₊₁ 516 whichis a measure of the amplitude of the +1st acoustic diffraction order530. In other examples, the detectors may be configured to detect phaseor direction of the diffracted acoustic orders.

Although overlay may be extracted from the asymmetry between thecorresponding opposite acoustic diffraction orders, overlay is just oneof the parameters which can affect the structural asymmetry. Variousfeatures of a grating or differences between one or more gratings mayresult in an acoustic diffraction order asymmetry. The features ordifferences may be physical shape differences such as sidewall angle,floor tilt or top tilt. The features or differences may beprocess-induced by variation of process parameters related to pitch,focus, or dose for example. Measurement of these process parametersthrough the asymmetry of the acoustic diffraction orders allows controlof the process to mitigate the process-induced variation. The grating orgratings may be one-dimensional (e.g. a set of parallel lines) ortwo-dimensional (e.g. an array of dots).

Although in the examples described herein, with reference to FIGS. 5 and12 to 18, opposite acoustic diffraction orders are first orders, otherorders may be used, for example the second or third order. Orders may becombined together, for example, the opposite diffraction orders may be−1st and −2nd on the one hand and +1st and +2nd orders on the otherhand.

The periodic structure is now described in more detail. As depicted incross-section in FIG. 5, it has a grating 538 overlaid over anothergrating 540, separated by a layer 534. The layer 534 may be opaque tooptical radiation, but transmits acoustic radiation.

The output intensity signals I⁻¹ 514 and I₊₁ 516 are received by aprocessing unit 506. The signals are subtracted by a subtraction block510. A post-processing block 508 multiplies the subtracted delta(I⁻¹-I₊₁) by 1/K to obtain the overlay error OV 504. Although in thisexample a digital processing unit is used, analogue circuitry may beused to determine the overlay and/or other property of the substrate.

K may be determined using the following methods known from opticalmetrology.

A model of the structure may be used. For example, a “nominal model”,which in essence is a theoretical best guess of what the target ordevice structure is, may be used. This theoretical description may bedetermined from a full reconstruction of the structure using Maxwell'sequation, or from SEM measurements. The response of the structure forthe particular wavelength or other illumination conditions is simulated,and K is determined. In this way, the proportionality constant K can bedetermined by computing the overlay sensitivity (K=dI/dOV) for thenominal stack. Here ‘nominal stack’ represents the best availableknowledge about the actual stack of layers in the semiconductor processthat are used to fabricate the structure.

Another option is to determine K experimentally via set/get experiments.Various induced overlay values are fabricated. A multitude of targetswith programmed overlays are printed (set wafers) and overlaysensitivity K is measured on the wafers (intensity variation as afunction of overlay).

Again as known from optical overlay metrology, +/−d biased targets maybe used to determine K, where d is a deliberate overlay offset. Thisleads to two equations where K and overlay OV are unknown, and asymmetryof intensity and d (bias) are known. K may be determined using thesebiased targets. One can also measure overlay OV directly without knowingK.

In the example of FIG. 5, the asymmetry of amplitude is used, bysubtracting intensity signals from the acoustic detectors, as follows.In other embodiments, the acoustic direction may be detected or angulardirection may be detected and used to determine the overlay and/or otherproperty of the substrate. At this frequency range phase is stillmeasurable with transducers. The acoustic direction can be deduced by adifferent propagation path which can be measured by comparing the phaseof the incoming pulse with respect to the received echo pulse.

The alternatives (including various measured parameters, substrateproperties and diffraction orders) discussed with reference FIG. 5 abovealso apply to the other examples described herein, with reference toFIGS. 12 to 18.

FIGS. 6 and 7 depict a simulated pressure distribution for a symmetricalstructure 602 and an asymmetrical structure 702 respectively. These arethe results for simulations for a realistic overlapping grating (usingthe K-wave toolbox for MATLAB™), wherein one can identify that anoverlay error between the grating manifests itself as an asymmetry 704in the diffracted acoustic wavefront, compared to the symmetrical case604. These particular simulations are in transmission, but a similareffect is encountered in reflection as well.

1. Summary

In Sections 2 to 8 below, a more rigorous approach is described toquantify the asymmetry between diffracted acoustic orders in the casethat overlay is present.

In summary, the findings are:

Overlay creates a diffracted orders asymmetry;

All diffracted acoustic waves from an overlapping grating will becharacterized by two propagation velocities in the material,corresponding to compression and shear waves—thus we will have threeinterfering waves in the signal formation model for the first (orhigher) orders. The overlay signal is shown to be proportional to theintensity difference between the diffracted orders;

The overlay sensitivity depends on two sine waves with differentfrequencies; and

In order to estimate the sensitivity ΔI/OV an estimate of the reflectioncoefficient is needed. An alternative is to assume 10% reflection fromeach grating, making the overlay signal of the order of 1% (1×10⁻²) fromthe input signal.

2. Introduction

The purpose of the following is to derive an acoustic equivalent of thesimplified signal formation model in optical diffraction-based overlay(DBO) measurements. Sections 3-5 contain a minimum of required theory inorder to arrive at plane-wave solutions for an acoustic scatteringproblem. Section 6 describes the classical signal formation model foroptical DBO measurements. Finally, Section 7 uses the theory fromSections 3-5 in order to derive a similar model for acoustics. The maindifference with respect to the optics model is the presence in solidmaterials of two waves travelling with different velocities. Theconsequence is a slightly more complex ‘swing curve’ for the overlaysensitivity.

3. Acoustic Wave Equation

The acoustic wave equation in homogeneous materials is given by

ρ∂u=(λ+2μ)∇(∇·u)−μ∇×(∇×u).  (1)

where u=[u_(x), u_(y), u_(z)]^(T) is the displacement vector, λ>0 isLamé's first parameter, μ≥0 is Lamé's second parameter (shear modulus),ρ>0 is the material density. The shear modulus is zero for fluids andpositive for solid materials. Note that the above equation is derivedunder the assumption of homogeneous and isotropic materials.

4. Compression Waves and Shear Waves

The displacement u is often expressed in terms of the compression wave(or P-wave) scalar potential p and shear wave (or S-wave) vectorpotential s. The reason for this particular naming will become obviousin the next section. We will use the Helmholtz decomposition theorem

u=u _(p) +u _(s) =∇p+∇×s, ∇·s=0.  (2)

Note that

∇×∇p=0,  (3)

∇·(∇×s)=0.  (4)

Substitute (2) in (1),

ρ∂_(tt)(∇p + ∇ × s) = (λ + 2μ)∇(∇⋅(∇p + ∇×s)) − μ∇×(∇×(∇p + ∇ × s)) = (λ + 2μ)∇(∇⋅∇p) − μ∇×(∇×(∇×s)) = (λ + 2μ)∇(∇²p) + μ∇²(∇ × s)

where in the last step we have used the fact that

∇·(∇p)=∇² p,  (5)

−∇×(∇×∇×s)=∇²(∇×s).  (6)

Grouping the terms together we obtain

∇(ρ∂_(tt) p−(λ+2μ)∇² p)=∇×(ρ∂_(tt) s−μ∇ ² s).

The left-hand side is the gradient of a function of p, while theright-hand side is the curl of a function of s. As these sides arealways equal for all t and x, they must be equal to some constant, whichwe can take as zero. Thus, the Helmholtz decomposition has enabled us toseparate the elastodynamic equation of motion for an isotropic mediuminto two differential equations:

ρ∂_(tt) p=(λ+2μ)∇² p,  (7)

ρ∂_(tt) s=μ∇ ² s.  (8)

These are wave equations with phase velocities. Note that the shear wavealways travels slower than the compression wave, c_(s)<c_(p). In thetime-harmonic case, the wave equation becomes a Helmholtz equation. Inoptics, the Helmholtz equation is the starting point for the scalartheory of diffraction and other simplified models. This implies thatsimilar models can be used in acoustics.

5. Plane Wave Solutions

In this section we will look at plane wave solutions of (7)-(8) in orderto understand their properties. Given a wave equation

$\begin{matrix}{{{\partial_{tt}w} = {\frac{1}{c^{2}}{\nabla^{2}w}}},} & (9)\end{matrix}$

its general plane-wave solution is given by

w(x,t)=re^(i(k·x±ωt)),  (10)

with

$\begin{matrix}{{k} = {\frac{2\pi}{\lambda} = {\frac{\omega}{c}.}}} & (11)\end{matrix}$

A plane-wave solution to the scalar wave equation for elastic media (7)is given by

p(x,t)=ae ^(i(k·x±ωt)).  (12)

To obtain the displacement field due to the scalar potential p, we applythe gradient

u _(p) =∇p=iake ^(i(k·x±ωt)).  (13)

This is a harmonic displacement disturbance with all displacement in thepropagation direction k and is therefore referred to as compression waveor P-wave.

A plane-wave solution to the vector wave equation for elastic media (8)is given by

$\begin{matrix}{{{s\left( {x,t} \right)} = {\begin{pmatrix}b_{x} \\b_{y} \\b_{z}\end{pmatrix}e^{i{({{k \cdot x} \pm {\omega \; t}})}}}}.} & (14)\end{matrix}$

To obtain the displacement field due to the vector potential s, we applythe curl

$\begin{matrix}{u_{s} = {{\nabla \times s} = {\begin{pmatrix}{{\partial_{y}s_{z}} - {\partial_{z}s_{y}}} \\{{\partial_{z}s_{x}} - {\partial_{x}s_{z}}} \\{{\partial_{x}s_{y}} - {\partial_{y}s_{x}}}\end{pmatrix} = {{i\begin{pmatrix}{{b_{z}k_{y}} - {b_{y}k_{z}}} \\{{b_{x}k_{z}} - {b_{z}k_{x}}} \\{{b_{y}k_{x}} - {b_{x}k_{y}}}\end{pmatrix}}{e^{i{({{k \cdot x} \pm {\omega \; t}})}}.}}}}} & (15)\end{matrix}$

This is a harmonic displacement disturbance with all displacementperpendicular to the propagation direction k and is therefore referredto as shear wave or S-wave. To check the orthogonality with respect tok, take the inner product,

$\begin{matrix}{{{- {i\begin{pmatrix}{{b_{z}k_{y}} - {b_{y}k_{z}}} \\{{b_{x}k_{z}} - {b_{z}k_{x}}} \\{{b_{y}k_{x}} - {b_{x}k_{y}}}\end{pmatrix}}^{T}} \cdot \begin{pmatrix}k_{x} \\k_{y} \\k_{z}\end{pmatrix}} = {{{b_{z}k_{x}k_{y}} - {b_{y}k_{x}k_{z}} + {b_{x}k_{y}k_{z}} - {b_{z}k_{x}k_{y}} + {b_{y}k_{x}k_{z}} - {b_{x}k_{y}k_{z}}} = 0.}} & (16)\end{matrix}$

6. Existing Signal Formation Model for Electromagnetic DBO

FIG. 8 depicts, in schematic form, a simplified model to explain signalformation for conventional optical diffraction-based overlay.

We consider the configuration from FIG. 8, where a plane-wave 826illuminates an overlay target 834 comprising two ‘overlayed’ gratings838 and 840 separated by a layer with thickness T. Under the assumptionof single scattering (Born approximation, low optical contrast) theintensity of the first orders is given by

I₊₁ = A e^(i α) + Be^(i β)² = (Ae^(i α) + Be^(i β))(A^(*)e^(−i α) + B^(*)e^(−i β)) = A² + B² + BA^(*)e^(i(β − α)) + AB^(*)e^(i(α − β))) = A² + B² + 2Re(AB^(*))cos (α − β).I⁻¹ = A e^(−i α) + Be^(i β)² = (Ae^(−i α) + Be^(i β))(A^(*)e^(i α) + B^(*)e^(−i β)) = A² + B² + BA^(*)e^(i(β + α)) + AB^(*)e^(−i(α − β))) = A² + B² + 2Re(AB^(*))cos (α + β).

The overlay is extracted from the intensity difference

Δ I = I₊₁ − I⁻¹ = 2Re(AB^(*))(cos (α − β) − cos (α + β)) = 4Re(AB^(*))sin (α)sin (β).

Substituting the values for α, β from FIG. 8,

$\begin{matrix}{{\alpha = {2\pi \frac{OV}{P}}},} & (17) \\{{\beta \approx {4\pi \frac{T}{\lambda}}},} & (18)\end{matrix}$yields

${\Delta I} = {{4R{e\left( {AB^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda}} \right)}{\sin \left( {2\pi \frac{OV}{P}} \right)}} \approx {8R{e\left( {AB^{*}} \right)}\frac{\pi}{P}{\sin \left( {4\pi \frac{T}{\lambda}} \right)}{{OV}.}}}$

The overlay sensitivity is then given by

${K = {\frac{\Delta I}{OV} \approx {8R{e\left( {AB^{*}} \right)}\frac{\pi}{P}{\sin \left( {4\pi \frac{T}{\lambda}} \right)}}}}.$

FIG. 9 is a graph of a conventional optical swing curve. FIG. 9 shows aqualitative swing curve, where we assumed

[AB*]=10⁻²,  (19)

P=1 μm,  (20)

T=0.2 μm.  (21)

Note that it is customary to assume the reflection coefficients to beconstant when plotting a qualitative swing curve. In reality thesecoefficients are wavelength-dependent and may have a significant impacton the final shape of the actual swing curve.

7. Proposed Signal Formation Model for an Acoustic DBO

FIG. 10 depicts, in schematic form a simplified model to explain signalformation for acoustic diffraction-based overlay. We consider theconfiguration from FIG. 10, where an acoustic plane-wave 1026 impingeson an overlay target 1035 comprising two ‘overlayed’ gratings 1038 and1040. The superstrate couplant material 1024 is fluid (gas or liquid)with μ=0 and the material between the gratings 1034 is a solid with μ>0and thickness T. Note that the choice of materials is critical as μ>0implies the presence of shear waves (next to compression waves). Thus,in the fluid-solid situation there are only compression waves in thefluid and both compression and shear waves in the solid. If thesuperstrate is replaced by a solid, or a very high-viscosity shear-wavecouplant, a shear wave would need to be taken into account in thesuperstrate as well.

Under the assumption of single scattering (Born approximation, lowacoustic contrast) the intensity of the first orders is given by

$\begin{matrix}{I_{+ 1} = {{{Ae^{i\; \alpha}} + {Be^{i\; \beta}} + {Ce^{i\; \gamma}}}}^{2}} \\{= {\left( {D + {Ce}^{i\; \gamma}} \right)\left( {D^{*} + {C^{*}e^{{- i}\; \gamma}}} \right)}} \\\left. {= {{D}^{2} + {C}^{2} + {D\; C^{*}e^{{- i}\; \gamma}} + {{CD}^{*}e^{i\; \gamma}}}} \right) \\{= {{A}^{2} + {B}^{2} + {2{{Re}\left( {AB}^{*} \right)}{\cos \left( {\alpha - \beta} \right)}} + {C}^{2} +}} \\{{{A\; C^{*}e^{i{({\alpha - \gamma})}}} + {{BC}^{*}e^{i{({\beta - \gamma})}}} + {{CA}^{*}e^{- {i{({\alpha - \gamma})}}}} + {{CB}^{*}e^{- {i{({\beta - \gamma})}}}}}} \\{= {{A}^{2} + {B}^{2} + {C}^{2} +}} \\{{{2{{Re}\left( {AB}^{*} \right)}{\cos \left( {\alpha - \beta} \right)}} + {2{{Re}\left( {A\; C^{*}} \right)}{\cos \left( {\alpha - \gamma} \right)}} +}} \\{{2{{Re}\left( {BC}^{*} \right)}{{\cos \left( {\beta - \gamma} \right)}.}}}\end{matrix}$ $\begin{matrix}{I_{- 1} = {{{Ae^{{- i}\; \alpha}} + {Be^{i\; \beta}} + {Ce^{i\; \gamma}}}}^{2}} \\{= {\left( {D + {Ce}^{i\; \gamma}} \right)\left( {D^{*} + {C^{*}e^{{- i}\; \gamma}}} \right)}} \\\left. {= {{D}^{2} + {C}^{2} + {D\; C^{*}e^{{- i}\; \gamma}} + {{CD}^{*}e^{i\; \gamma}}}} \right) \\{= {{A}^{2} + {B}^{2} + {2{{Re}\left( {AB}^{*} \right)}{\cos \left( {\alpha + \beta} \right)}} + {C}^{2} +}} \\{{{A\; C^{*}e^{i{({{- \alpha} - \gamma})}}} + {{BC}^{*}e^{i{({\beta - \gamma})}}} + {{CA}^{*}e^{i{({\alpha + \gamma})}}} + {{CB}^{*}e^{i{({{- \beta} + \gamma})}}}}} \\{= {{A}^{2} + {B}^{2} + {C}^{2} +}} \\{{{2{{Re}\left( {AB}^{*} \right)}{\cos \left( {\alpha + \beta} \right)}} + {2{{Re}\left( {A\; C^{*}} \right)}{\cos \left( {\alpha + \gamma} \right)}} +}} \\{{2{{Re}\left( {BC}^{*} \right)}{{\cos \left( {\beta - \gamma} \right)}.}}}\end{matrix}$

The overlay is extracted from the intensity difference

$\begin{matrix}{{\Delta \; I} = {I_{+ 1} - I_{- 1}}} \\{= {{2{{Re}\left( {AB^{*}} \right)}\left( {{\cos \left( {\alpha - \beta} \right)} - {\cos \left( {\alpha + \beta} \right)}} \right)} +}} \\{{2R{e\left( {AC^{*}} \right)}\left( {{\cos \left( {\alpha - \gamma} \right)} - {\cos \left( {\alpha + \gamma} \right)}} \right)}} \\{= {{4R{e\left( {AB^{*}} \right)}{\sin (\alpha)}{\sin (\beta)}} - {4R{e\left( {AC^{*}} \right)}{\sin (\alpha)}{\sin (\gamma)}}}} \\{= {4\left( {{R{e\left( {AB^{*}} \right)}{\sin (\beta)}} - {4R{e\left( {AC^{*}} \right)}{\sin (\gamma)}}} \right){{\sin (\alpha)}.}}}\end{matrix}$

Substituting the values for α, β, γ from FIG. 3,

$\begin{matrix}{{\alpha = {2\pi \frac{OV}{P}}},} & (22) \\{{\beta \approx {4\pi \frac{T}{\lambda_{p}}}},} & (23) \\{{\gamma \approx {4\pi \frac{T}{\lambda_{s}}}},} & (24)\end{matrix}$yields

$\begin{matrix}{{\Delta \; I} = {4\left( {{R{e\left( {AB^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{p}}} \right)}} - {R{e\left( {AC^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{s}}} \right)}}} \right){\sin \left( {2\pi \frac{OV}{P}} \right)}}} \\{\approx {\frac{8\pi}{P}\left( {{R{e\left( {AB^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{p}}} \right)}} - {R{e\left( {AC^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{s}}} \right)}}} \right){{OV}.}}}\end{matrix}$

The overlay sensitivity is then given by

${K = {\frac{\Delta I}{OV} \approx {\frac{8\pi}{P}\left( {{R{e\left( {AB^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{p}}} \right)}} - {R{e\left( {AC^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{s}}} \right)}}} \right)}}}.$

Interestingly, the ‘swing curve’ for acoustic overlay measurementconsists of two sines with different frequencies.

8. Estimates

Table 1 lists the material properties relevant for acoustic waves. Thefirst Lame coefficient λ is obtained from Young's modulus

${\lambda = \frac{vE}{\left( {1 + v} \right)\left( {1 - {2v}} \right)}},$

or from the bulk modulus

$\lambda = {K - {\frac{2}{3}{\mu.}}}$

TABLE 1 Material properties relevant for acoustic waves. Speed of soundin air: c = 340 m/s Material properties air: ρ = 1225 kg/m³, λ = K = 101kPa, μ = 0 Material properties SiO₂: ρ = 2203 kg/m³, λ = 16.09 GPa (E =73.1 GPa, v = 0.17), μ = 31.2 GPa Material properties Si: ρ = 2329kg/m³, λ = 93.46 GPa (E = 188 GPa, v = 0.28), μ = 80 GPa

Assuming a wavelength in air

Λ=0.1 μm,

the angular frequency is given by

$\omega = {\frac{2\pi c}{\Lambda} = {9{.32425}*10^{10}{s^{- 1}.}}}$

Velocity of P and S waves in silicon

$\begin{matrix}{{c_{{Si},p} = {\sqrt{\frac{\lambda + {2\mu}}{\rho}} = {8288.49\mspace{14mu} m\text{/}s}}},} & (25) \\{c_{{Si},s} = {\sqrt{\frac{\mu}{\rho}} = {586{0.8}5\mspace{14mu} m\text{/}{s.}}}} & (26)\end{matrix}$

Wavelength of P and S waves in silicon

$\begin{matrix}{{\Lambda_{{Si},P} = {\frac{2\pi c_{{Si},P}}{\omega} = {{0.5}6\mspace{11mu} {µm}}}},} & (27) \\{\Lambda_{{Si},S} = {\frac{2\pi c_{{Si},S}}{\omega} = {{0.3}9\mspace{14mu} {{µm}.}}}} & (28)\end{matrix}$

FIG. 11 is a graph of an acoustic swing curve. FIG. 11 shows aqualitative swing curve, where we assumed

[AB*]=10⁻²,  (29)

[AC]=10⁻²,  (30)

P=1 μm,  (31)

T=0.2 μm.  (32)

As for the optical case, we assumed the reflection coefficients to beconstant when plotting a qualitative acoustic swing curve. In realitythese coefficients are wavelength-dependent and may have a significantimpact on the final shape of the actual swing curve.

The modulus of the overlay sensitivity can be estimated for a best casescenario as follows

${K} = {{\frac{\Delta I}{OV}} \approx {\frac{8\pi}{P}{{{{{Re}\left( {AB^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{p}}} \right)}} - {R{e\left( {AC^{*}} \right)}{\sin \left( {4\pi \frac{T}{\lambda_{s}}} \right)}}}}} \leq {\frac{8\pi}{P}{\left( {{{{Re}\left( {AB^{*}} \right)}} + {{{Re}\left( {AC^{*}} \right)}}} \right).}}}$

The values A, B, C can be determined with a rigorous acoustic model.

FIG. 12 depicts an acoustic scatterometer with three acoustictransducers and a central acoustic source. This is the sameconfiguration as depicted in FIG. 5. It is reproduced here to depict theelements, which have the same reference numerals (beginning with 12),that are found in other examples depicted in FIGS. 13 to 16.

With reference to FIG. 12, the scatterometer has an acoustic source 1220operable to project acoustic radiation 1226 onto a periodic structure1238 formed on a substrate 1234. An acoustic detector 1218 is operableto detect the −1st acoustic diffraction order 1228 diffracted by theperiodic structure 1238 while discriminating from specular reflection(0th order 1232). Another acoustic detector 1222 is operable to detectthe +1st acoustic diffraction order 1230 diffracted by the periodicstructure, again while discriminating from the specular reflection (0thorder 1232). The acoustic source and acoustic detector may be piezotransducers as described with reference to FIG. 4.

The angle of incidence of the projected acoustic radiation 1226 andlocation of the detectors 1218 and 1222 are arranged with respect to theperiodic structure 1238 such that the detection of the −1st and +1stacoustic diffraction orders 1228 and 1230 discriminates from the 0thorder specular reflection 1232. The angle of incidence and positions maybe determined by simulation and/or experimentation and depend on thepitch of the gratings and wavelength of the acoustic radiation.

A couplant system provides a couplant 1224 to facilitate transmission ofthe acoustic radiation between the acoustic source 1220 and the acousticdetectors 1218, 1222 via the periodic structure 1238.

The acoustic detector 1218 outputs an intensity signal I⁻¹ 1214 which isa measure of the −1st acoustic diffraction order 1228. The acousticdetector 1222 outputs an intensity signal I₊₁ 1216 which is a measure ofthe +1st acoustic diffraction order 1230.

The output intensity signals I⁻¹ 1214 and I₊₁ 1216 are received by aprocessing unit 1206, which operates in the same way as described withreference to processing unit 506 described with reference to FIG. 5.

FIG. 13 depicts an acoustic scatterometer implemented with a phasedarray of transducers. The sub-elements of the phased array are splitinto three groups 1318, 1320 and 1322, acting as a central acousticsource 1320 between a pair of acoustic detectors 1318 and 1322.

The operation is the same as described with reference to FIG. 12 for theelements with common reference numerals, with the following differences.Acoustic detector 1318 is arranged to receive an acoustic diffractionorder 1228 diffracted by the periodic structure 1238. Acoustic detector1322 is arranged to receive another acoustic diffraction order 1230diffracted by the periodic structure that is opposite with respect tothe acoustic diffraction order received by the first acoustic detector1318.

FIGS. 14a and 14b depict an acoustic scatterometer with three acoustictransducers. There are two acoustic sources 1418, 1422 and a centralacoustic detector 1420. The operation is the same as described withreference to FIG. 12 for the elements with common reference numerals,with the following differences.

A first acoustic source 1418 is arranged to project (FIG. 14a ) firstacoustic radiation 1426 onto a periodic structure 1238 with a firstangle of incidence. A second acoustic source 1422 is arranged to project(FIG. 14b ) second acoustic radiation 1480 onto a periodic structure1238 with a second angle of incidence.

In some examples (not shown) the first and second angles of incidenceneed not be equal. A detector with a wide acceptance angle may be used,or even separate detectors to detect diffraction orders from the firstand second acoustic sources. Thus there may be four or more transducers,for example two acoustic sources and two acting as acoustic detectors.

In this example, the first and second acoustic sources are configurableto project their acoustic radiation at different times t=t₀ (FIG. 14a )and t=t₁ (FIG. 14b ). The acoustic radiation is projected at thedifferent times with first and second angles of incidence selected suchthat an acoustic detector 1420 is configurable to separately detect +1st(FIG. 14a ) and −1st (FIG. 14b ) acoustic diffraction orders.

In FIG. 14a , the acoustic detector 1420 is configured to detect the+1st diffraction order 1426, which has been diffracted by the periodicstructure 1238 and arising from irradiation 1428 by the first acousticsource 1418. The acoustic detector 1420 outputs an intensity signal I₊₁1416 which is a measure of the +1st acoustic diffraction order 1426.

In FIG. 14b , the acoustic detector 1420 is configured to detect anotheracoustic diffraction order 1476, which has been diffracted by theperiodic structure 1238 and arises from irradiation 1480 by the secondacoustic source 1422. Diffraction order 1476 is opposite (−1st order)with respect to the acoustic diffraction order 1426 (+1st order) arisingfrom irradiation 1428 by the first acoustic source 1418. The acousticdetector 1420 outputs an intensity signal I⁻¹ 1464 which is a measure ofthe −1st acoustic diffraction order 1476.

The output intensity signals I⁻¹ 1464 and I₊₁ 1416 are received by aprocessing unit 1206, which operates in the same way as described withreference to processing unit 506 described with reference to FIG. 5.

The acoustic source in the examples depicted in FIGS. 5, 12, 13 and 14are operable to project continuous wave acoustic radiation onto theperiodic structure while the acoustic detector is operable tosimultaneously detect the acoustic diffraction order diffracted by theperiodic structure. This is allowed because the acoustic source andacoustic detector are not the same transducer.

FIGS. 15a and 15b depict an acoustic scatterometer with two transducersoperating as transceivers. The operation is the same as described withreference to FIG. 12 for the elements with common reference numerals,with the following differences. An acoustic transceiver 1518 isconfigurable as the acoustic source (in FIG. 15a ) to project pulsedacoustic radiation 1526 and configurable as the acoustic detector (inFIG. 15a ) to detect a backscattered acoustic diffraction order 1528diffracted by the periodic structure 1238 and arising from the pulsedacoustic irradiation 1526. The acoustic transceiver 1518 outputs anintensity signal I⁻¹ 1514 which is a measurement of the −1st acousticdiffraction order 1528.

In this example with a pair of transceivers, the other acoustictransceiver 1520 is configurable as the acoustic source (in FIG. 15b )to project pulsed acoustic radiation 1576 and configurable as theacoustic detector (in FIG. 15b ) to detect a backscattered acousticdiffraction order 1578 diffracted by the periodic structure 1238 andarising from the pulsed acoustic irradiation 1576. The acoustictransceiver 1520 outputs an intensity signal I₊₁ 1566 which is ameasurement of the +1st acoustic diffraction order 1578.

In this example, the acoustic transceivers 1518 and 1520 areconfigurable to project their acoustic radiation at different times t=t₀(FIG. 15a ) and t=t₁ (FIG. 15b ), so that the backscattered acousticdiffraction order at a transceiver can be discriminated from thespecular reflection arising from the irradiation 1532, 1582 by the othertransceiver.

The output intensity signals I⁻¹ 1514 and I₊₁ 1566 are receivedsequentially by a processing unit 1206, which operates in the same wayas described with reference to processing unit 506 described withreference to FIG. 5.

FIGS. 16a and 16b depict an acoustic scatterometer with one transduceroperating as a transceiver. The operation is the same as described withreference to FIG. 12 for the elements with common reference numerals,with the following differences. An acoustic transceiver 1618 isconfigurable as the acoustic source (in FIG. 16a ) to project pulsedacoustic radiation 1626 and configurable as the acoustic detector (inFIG. 16a ) to detect a backscattered acoustic diffraction order 1628diffracted by the periodic structure 1238 and arising from the pulsedacoustic irradiation 1626. The acoustic transceiver 1618 outputs anintensity signal I⁻¹ 1614 which is a measurement of the −1st acousticdiffraction order 1628.

In this example with a single transceivers, the periodic structure orthe detector are rotated by 180 degrees around the directionperpendicular to the direction of periodicity of the periodic structurebetween measurement of the opposing acoustic diffraction orders.Alternatively, the transceiver may be simply moved along another pathand redirected at the target. After the rotation or movement, thetransceiver 1618 is configurable as the acoustic source (in FIG. 16b )to project pulsed acoustic radiation 1676 and configurable as theacoustic detector (in FIG. 16b ) to detect a backscattered acousticdiffraction order 1680 diffracted by the periodic structure 1238 andarising from the pulsed acoustic irradiation 1676. The acoustictransceiver 1618 outputs an intensity signal I₊₁ 1666 which is ameasurement of the +1st acoustic diffraction order 1680.

The output intensity signals I⁻¹ 1614 and I₊₁ 1666 are receivedsequentially by a processing unit 1206, which operates in the same wayas described with reference to processing unit 506 described withreference to FIG. 5.

FIG. 17 depicts a method of scatterometry. This method may beimplemented using the scatterometers described with reference to FIGS.5, 12 and 13.

With reference to FIG. 17, a method of scatterometry comprises:

1702 (Tx): projecting acoustic radiation onto a periodic structureformed on a substrate;

1704 (Rx): detecting an acoustic diffraction order diffracted by theperiodic structure while discriminating from specular reflection; and

1706 (CALC): determining a property of the substrate based on thedetected acoustic diffraction order.

The angle of incidence of the projected acoustic radiation and locationof the detector with respect to the periodic structure are arranged suchthat the detection of the acoustic diffraction order discriminates fromspecular reflection. A couplant is provided to facilitate transmissionof the acoustic radiation between the acoustic source and the acousticdetector via the periodic structure.

The property of the substrate may be determined based on at least one ofamplitude, phase and direction of one or more detected acousticdiffraction order. The property of the substrate may comprise a physicalproperty of the periodic structure.

The method may comprising determining a structural asymmetry of theperiodic structure based on asymmetry of at least two correspondingopposite detected acoustic diffraction orders diffracted by the periodicstructure.

The periodic structure may comprise a grating overlaid over anothergrating and the method may comprise determining an overlay error basedon asymmetry of at least two corresponding opposite detected acousticdiffraction orders diffracted by the gratings.

The overlay error may be determined based on asymmetry of at least oneof amplitude, phase and direction of the at least two correspondingopposite detected acoustic diffraction orders diffracted by thegratings.

FIG. 18 depicts a method including projecting acoustic radiation atdifferent times and with first and second angles of incidence, inaccordance with an embodiment of the present invention. This method maybe implemented using the scatterometers described with reference toFIGS. 14 to 16, and could even be implemented using the scatterometersdescribed with reference to FIGS. 5, 12 and 13.

With reference to FIG. 18, acoustic radiation may be projected atdifferent times 1802 (Tx1), 1806 (Tx2) and with respective first andsecond angles of incidence selected to separately detect:

1804 (Rx1): an acoustic diffraction order (e.g. +1) diffracted by theperiodic structure and arising from irradiation at the first angle ofincidence; and

1808 (Rx2): another acoustic diffraction order (e.g. −1) diffracted bythe periodic structure that is opposite with respect to the acousticdiffraction order (e.g. +1) arising from irradiation at the first angleof incidence.

At step 1808 (CALC) a property of the substrate is determined based onthe detected acoustic diffraction order.

The methods of FIGS. 17 and 18 may comprise projecting continuous waveacoustic radiation onto the periodic structure while detecting theacoustic diffraction order diffracted by the periodic structure.

The methods of FIGS. 17 and 18 may comprise projecting pulsed acousticradiation and detecting a backscattered acoustic diffraction orderdiffracted by the periodic structure and arising from the projection ofthe pulsed acoustic irradiation.

Embodiments of the present invention provide a non-destructive and fastway to measure properties of periodic structures.

In a further embodiment of the present invention, a measurement of onlythe zeroth order or the specular reflected light may be used to providea measurement of a parameter of interest, in particular a parameter ofinterest such as overlay. In an embodiment, an acoustic polarizingelement may be used. An acoustic polarizing element may be a singletransducer or a an array of 2D transducers, In an example, thetransducer 1220 of FIG. 12 may polarize the acoustic radiation in onepolarizing direction and the transducers 1218 or 1222 of FIG. 12 maycomprise an acoustic polarizing element in a direction orthogonal to thepolarizing direction of transducer 1220. Such a setup is called a crosspolarizing setup. When zeroth order radiation or specular radiation isused in a cross polarizing setup, a measurement of parameter of interestrepresentative of product structures on a wafer may be measured.

Scatterometry using zeroth order electromagnetic radiation is known inthe art. A parameter of interest of a lithographic process, such asoverlay, may be measured by observing the asymmetry formed in theasymmetric pupil. If acoustic radiation is used, the principle ofmeasurement remains the same, although the components involved informing the overlay signal are different, as the physics of the acousticradiation and the scattering thereof is different from the physics ofelectromagnetic radiation and the scattering thereof. For an acousticsetup, such as the one depicted in FIG. 12 for example, the acousticJones matrix may be expressed as

$J_{AB} = \begin{pmatrix}R_{PP} & R_{PH} & R_{PV} \\R_{HP} & R_{HH} & R_{HV} \\R_{VP} & R_{VH} & R_{VV}\end{pmatrix}$

wherein the left side is the acoustic Jones matrix and the components ofthe right hand matrix are the reflection coefficients for an incidentradiation with a polarization direction and outgoing radiation with adifferent or the same polarization direction of the acoustic radiation.In the case of acoustic radiation, the light may be polarized along theshear or compression waves direction. Thus, for acoustic metrology, oneneeds to consider shear polarized radiation or compression polarizedradiation. The principle of measurement overlay with electromagneticradiation apply also to the measurement of overlay with acousticradiation, with the difference that, in the acoustic metrology setup oneneeds to consider 3 polarization direction, and the combination thereof,as opposed to the case of electromagnetic radiation, wherein only 2polarization directions are needed. The setup for acoustic metrologydescribed in previous embodiments is also applicable for measurements inan acoustic cross polarizing setup with the difference that zeroth orderof the acoustic radiation is detected and not the diffracted orders ofthe acoustic radiation. Acoustic polarized radiation may be created withknown methods, as state of the art comprises solutions to forming bothshearing and compressing polarized acoustic radiation.

Scatterometers in accordance with embodiments of the present inventionmay be used where optical scatterometers are not suitable.Alternatively, scatterometers in accordance with embodiments of thepresent invention may be used to complement an optical scatterometer inan inspection apparatus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the scatterometer described herein may have otherapplications. Possible other applications include the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, flat-panel displays, liquid-crystal displays (LCDs),thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a wafer inspection apparatus,embodiments of the invention may be used in other apparatus. Embodimentsof the invention may form part of a mask inspection apparatus, ametrology apparatus, or any apparatus that measures or processes anobject such as a wafer (or other substrate) or mask (or other patterningdevice). These apparatus may be generally referred to as lithographictools.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

Further embodiments according to the invention are further described inbelow numbered clauses:

1. A scatterometer comprising:

-   -   an acoustic source operable to project acoustic radiation onto a        periodic structure formed on a substrate; and    -   an acoustic detector operable to detect an acoustic diffraction        order diffracted by the periodic structure while discriminating        from specular reflection,    -   wherein the scatterometer is operable to determine a property of        the substrate based on the detected acoustic diffraction order.

2. The scatterometer of clause 1 wherein the angle of incidence of theprojected acoustic radiation and location of the detector are arrangedwith respect to the periodic structure such that the detection of theacoustic diffraction order discriminates from specular reflection.

3. The scatterometer of clause 1 or clause 2 further comprising acouplant system to provide a couplant to facilitate transmission of theacoustic radiation between the acoustic source and the acoustic detectorvia the periodic structure.

4. The scatterometer of any preceding clause wherein the scatterometeris operable to determine the property of the substrate based on at leastone of amplitude, phase and direction of one or more detected acousticdiffraction order.

5. The scatterometer of any preceding clause wherein the property of thesubstrate comprises a physical property of the periodic structure.

6. The scatterometer of any preceding clause wherein the scatterometeris operable to determine a structural asymmetry of the periodicstructure based on asymmetry of at least two corresponding oppositedetected acoustic diffraction orders diffracted by the periodicstructure.

7. The scatterometer of any preceding clause wherein the periodicstructure comprises a grating overlaid over another grating and thescatterometer is operable to determine an overlay error based onasymmetry of at least two corresponding opposite detected acousticdiffraction orders diffracted by the gratings.

8. The scatterometer of clause 7 wherein the overlay error is determinedbased on asymmetry of at least one of amplitude, phase and direction ofthe at least two corresponding opposite detected acoustic diffractionorders diffracted by the gratings.

9. The scatterometer of any preceding clause comprising:

-   -   a first acoustic detector arranged to receive an acoustic        diffraction order diffracted by the periodic structure; and    -   a second acoustic detector arranged to receive another acoustic        diffraction order diffracted by the periodic structure that is        opposite with respect to the acoustic diffraction order received        by the first acoustic detector.

10. The scatterometer of any of preceding clause comprising:

-   -   a first acoustic source arranged to project first acoustic        radiation onto a periodic structure with a first angle of        incidence; and    -   a second acoustic source arranged to project second acoustic        radiation onto a periodic structure with a second angle of        incidence.

11. The scatterometer of clause 10 wherein the first and second acousticsources are configurable to project their acoustic radiation atdifferent times and with first and second angles of incidence selectedsuch that an acoustic detector is configurable to separately detect:

-   -   an acoustic diffraction order diffracted by the periodic        structure and arising from irradiation by the first acoustic        source; and    -   another acoustic diffraction order diffracted by the periodic        structure that is opposite with respect to the acoustic        diffraction order arising from irradiation by the first acoustic        source.

12. The scatterometer of any preceding clause, wherein the acousticsource is operable to project continuous wave acoustic radiation ontothe periodic structure while the acoustic detector is operable to detectthe acoustic diffraction order diffracted by the periodic structure.

13. The scatterometer of any of clauses 1 to 11 comprising an acoustictransceiver configurable as the acoustic source to project pulsedacoustic radiation and configurable as the acoustic detector to detect abackscattered acoustic diffraction order diffracted by the periodicstructure and arising from the pulsed acoustic irradiation.

14. A method of scatterometry comprising:

-   -   projecting acoustic radiation onto a periodic structure formed        on a substrate;    -   detecting an acoustic diffraction order diffracted by the        periodic structure while discriminating from specular        reflection; and    -   determining a property of the substrate based on the detected        acoustic diffraction order.

15. The method of clause 14 comprising arranging the angle of incidenceof the projected acoustic radiation and location of the detector withrespect to the periodic structure such that the detection of theacoustic diffraction order discriminates from specular reflection.

16. The method of clause 14 or clause 15 further comprising providing acouplant to facilitate transmission of the acoustic radiation betweenthe acoustic source and the acoustic detector via the periodicstructure.

17. The method of any of clauses 14 to 16 comprising determining theproperty of the substrate based on at least one of amplitude, phase anddirection of one or more detected acoustic diffraction order.

18. The method of any of clauses 14 to 17 wherein the property of thesubstrate comprises a physical property of the periodic structure.

19. The method of any of clauses 14 to 18 comprising determining astructural asymmetry of the periodic structure based on asymmetry of atleast two corresponding opposite detected acoustic diffraction ordersdiffracted by the periodic structure.

20. The method of any of clauses 14 to 19 wherein the periodic structurecomprises a grating overlaid over another grating and the methodcomprises determining an overlay error based on asymmetry of at leasttwo corresponding opposite detected acoustic diffraction ordersdiffracted by the gratings.

21. The method of clause 20 comprising determining the overlay errorbased on asymmetry of at least one of amplitude, phase and direction ofthe at least two corresponding opposite detected acoustic diffractionorders diffracted by the gratings.

22. The method of any of clauses 14 to 21 comprising projecting acousticradiation at different times and with first and second angles ofincidence selected to separately detect:

-   -   an acoustic diffraction order diffracted by the periodic        structure and arising from irradiation at the first angle of        incidence; and    -   another acoustic diffraction order diffracted by the periodic        structure that is opposite with respect to the acoustic        diffraction order arising from irradiation at the first angle of        incidence.

23. The method of any of clauses 14 to 21 comprising projectingcontinuous wave acoustic radiation onto the periodic structure whiledetecting the acoustic diffraction order diffracted by the periodicstructure.

24. The method of any of clauses 14 to 22 comprising projecting pulsedacoustic radiation and detecting a backscattered acoustic diffractionorder diffracted by the periodic structure and arising from theprojection of the pulsed acoustic irradiation.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

1. A scatterometer comprising: an acoustic source configured to projectacoustic radiation at an angle of incidence onto a target formed on asubstrate to produce first and second acoustic diffraction orders; afirst acoustic detector configured to detect the first acousticdiffraction order while discriminating from spectral reflection and toproduce a first detection signal; and a second acoustic detectorconfigured to detect the second acoustic diffraction order whilediscriminating from spectral reflection and to produce a seconddetection signal, wherein the scatterometer is configured to determine aproperty of the substrate based on the first and second detectionsignals, and wherein the target comprises: a first periodic structure; asecond periodic structure overlaying the first periodic structure; and alayer disposed between the first and second periodic structures.
 2. Thescatterometer of claim 1, wherein the layer transmits acoustic radiationbut is opaque to optical radiation.
 3. The scatterometer of claim 1,wherein the angle of incidence of the projected acoustic radiation andlocation of the first and second acoustic detectors are arranged withrespect to the target such that the detection of the first and secondacoustic diffraction orders discriminates from specular reflection. 4.The scatterometer of claim 1, further comprising a couplant configuredto facilitate transmission of the acoustic radiation between theacoustic source and the first and second acoustic detectors via thetarget.
 5. The scatterometer of claim 4, wherein the couplant comprisesdistilled water.
 6. The scatterometer of claim 1, wherein the acousticsource and the first and second acoustic detectors comprise piezotransducers.
 7. The scatterometer of claim 1, wherein the scatterometeris configured to determine the property of the substrate based on atleast one of amplitude, phase, and direction of the first and secondacoustic diffraction orders.
 8. The scatterometer of claim 1, whereinthe property of the substrate comprises a physical property of thetarget.
 9. The scatterometer of claim 8, wherein the physical propertycomprises an overlay error.
 10. The scatterometer of claim 1, whereinthe scatterometer is configured to determine a structural asymmetry ofthe target based on asymmetry of the detected first and second acousticdiffraction orders diffracted by the target.
 11. A scatterometercomprising: a first acoustic transceiver comprising: a first acousticsource configured to project first acoustic radiation at a first angleof incidence onto a target formed on a substrate to produce a firstacoustic diffraction order; and a first acoustic detector configured todetect the first acoustic diffraction order while discriminating fromspectral reflection and to produce a first detection signal; and asecond acoustic transceiver comprising: a second acoustic sourceconfigured to project second acoustic radiation at a second angle ofincidence onto the target to produce a second acoustic diffractionorder; and a second acoustic detector configured to detect the secondacoustic diffraction order while discriminating from spectral reflectionand to produce a second detection signal, wherein the scatterometer isconfigured to determine a property of the substrate based on the firstand second detection signals, and wherein the first and second acoustictransceivers are configured to project the first and second acousticradiation at different times and with the first and second angles ofincidence selected such that the first and second acoustic transceiversare configured to separately detect the first and second acousticdiffraction orders.
 12. The scatterometer of claim 11, furthercomprising a processor configured to sequentially receive first andsecond detection signals.
 13. The scatterometer of claim 11, furthercomprising a couplant configured to facilitate transmission of the firstand second acoustic radiation between the first and second acoustictransceivers via the target.
 14. The scatterometer of claim 11, whereinthe target comprises: a first periodic structure; a second periodicstructure overlaying the first periodic structure; and a layer disposedbetween the first and second periodic structures.
 15. The scatterometerof claim 11, wherein the property of the substrate comprises a physicalproperty of the target.
 16. A scatterometer comprising: an acoustictransceiver comprising: an acoustic source configured to project firstacoustic radiation at a first angle of incidence onto a target formed ona substrate to produce a first acoustic diffraction order, andconfigured to project second acoustic radiation at a second angle ofincidence onto the target to produce a second acoustic diffractionorder; and an acoustic detector configured to detect the first acousticdiffraction order while discriminating from spectral reflection and toproduce a first detection signal, and configured to detect the secondacoustic diffraction order while discriminating from spectral reflectionand to produce a second detection signal, wherein the scatterometer isconfigured to determine a property of the substrate based on the firstand second detection signals, and wherein the acoustic transceiver isconfigured to project the first and second acoustic radiation atdifferent times and with the first and second angles of incidenceselected such that the acoustic transceiver is configured to separatelydetect the first and second acoustic diffraction orders.
 17. Thescatterometer of claim 16, wherein the target or the acoustictransceiver are rotated by 180 degrees about an axis perpendicular to adirection of periodicity of the target in between the different timesthe first and second acoustic radiation are projected.
 18. Thescatterometer of claim 16, further comprising a processor configured tosequentially receive first and second detection signals.
 19. Thescatterometer of claim 16, further comprising a couplant configured tofacilitate transmission of the first and second acoustic radiationbetween the first and second acoustic transceivers via the target. 20.The scatterometer of claim 16, wherein the property of the substratecomprises a physical property of the target.